Red Clouds
As we all know the forest is usually covered in fog. The denseness of it is what varies. We also know the weather is uncontrollable here, despite every pegasi's efforts. Thus it's not surprising that even the weather would have it's own urban legends. This one specifically is about a red mist that covers a certain section of the woods.
Supposedly, this cloud comes at random, it's also rare, surprisingly. Maybe one of the legends that's actual legend. Though, there's just too many who reported it to push it over your shoulder. It seemingly comes from the trees, but many have all said that perhaps that was simply a trick of their eyes. Though, the further they walk their path the more dense it becomes.
Until eventually it's such a deep red that you're not sure if it's simply differently hued fog anymore. But like with normal fog, you can only make out basic figures now. When the fog is this dense, the smell is overwhelming and triggers your gag reflex according to survivors.
The smell is apparently different with every pony, probably the smell of what disgusts you. The type of smell that lingers at the back of your throat for days, only to transfer to taste. Some survivors actually vomited. Regardless of if you throw up or not, staying too long in this fog will make you dizzy, your vision hazy, and just generally weak feeling.
The problem is, is that you can't just escape this mist that easily. The forest either changes around you, to where you walk in circles in this fog without realizing or the fog encased you in it itself. Your eyes eventually water even, from how intense this smog can get.
Survivors reported that they experienced a harsh coughing spell, to the point that some had to stop walking to cough up their lungs. ..That's an expression this time. Though, all have said that after this, they coughed up saliva. Or at least they'd like to hope it's their saliva. With how red and thick the fog around them is, they couldn't tell if it was actual blood or not.
Coughing up saliva or blood, made them aware of the other places they were leaking liquids. Their noses, their eyes, and suddenly paranoid, they would all check if it's blood. Again, they couldn't tell. They couldn't even smell it with the overwhelming disgusting scent.
Uncertain panic welling up inside them, survivors often quickened their pace to get out of the cloud. The extra output of energy while already feeling physically weak caused a lot of them to collapse. And ultimately fall into a painful coughing fit. Those that pushed through and crawled or continued to walk through it managed to escape the red clouds eventually. To those that didn't, well.. It's assumed they succumbed to their coughing spells and might have actually coughed up a lung.
Those that survived had to traverse the remaining forest with weakened bodies. To be later hospitalized for a few months if they actually exited the forest. It seems that they actually did bleed internally to a point that it did in fact leak out of their eye sockets, mouths, noses and even their ears.
It's speculated that the only reason the fog is red is because it drains travelers of their blood supply. And the disgusting smell is what other ponies smelled like as they died inside of it. After all, the smell of death is never a pleasant one. That, or it's the workings of the forest to inflict such internalized deaths.